Devices such as optical, mechanical, electrical, and magnetic tachometers are used in the prior art to synchronize the interrogating signal and the rotating temperature sensors that must receive the signal. In many cases however a tachometer is not available or cannot be used because of unfavorable environmental and physical conditions.
For example, sensors can be placed on a gear inside a gear box where an interrogating antenna is stationary inside the gear box. As the gear rotates the sensors move into proximate relationship to the rotating sensor(s). The interrogating antenna is connected to an RF (radio frequency) interrogator, i.e., the device that issues the interrogating RF signal, with an RF feed through a hole in the gear box housing. The space between the gear and the gear box housing is limited and thus the interrogating antenna and the rotating sensor(s) are in a line-of-sight orientation only once during each revolution. This line-of-sight alignment is of course a random event, may change every revolution, and is not possible unless the interrogator has information related to the position of the rotating sensor. When a tachometer is not available such a wireless temperature sensing system is not reliable.
Another disadvantage of prior art shaft position sensors is their susceptibility to wear, tear, and misalignment, thereby reducing the reliability and integrity of the temperature sensing system.
A prior art SAW system comprises an interrogator further comprising a transmitter, receiver, and stationary antenna. The transmitter generates an RF interrogating signal or pulse at a SAW synchronous frequency or frequencies with an appropriate bandwidth and power level. The interrogating signal is directed toward the SAW substrate on which are mounted a SAW transducer and a reflector array. The transducer receives the interrogating RF signal at the SAW antenna, launches surface acoustic waves responsive thereto, receives reflected surface acoustic waves produced by a reflector array, and generates an RF echo signal or pulse responsive to the reflected surface acoustic waves. The RF echo signal is transmitted back to the interrogator via the SAW antenna where the receiver component detects and processes the echoes.
FIG. 1 depicts such a prior art SAW sensor system 8. An interrogator 10, under control of a user controller 11, generates and transmits an RF interrogating signal 12 from an antenna 13.
The RF interrogating signal is received by an antenna 18 connected to an interdigital transducer (IDT) 20 disposed on a piezoelectric substrate 24. The IDT 20 typically comprises positive electrodes connected to a positive bus bar 25 and negative electrodes connected to a negative bus bar 26. Terminals of the antenna 18 are connected to the positive and negative bus bars 25 and 26. Responsive to the RF interrogating signal, the IDT 20 launches incident surface acoustic waves (SAW) 28 onto the piezoelectric substrate 24. The SAW is launched from both sides of the IDT 20 and travels along the surface of the piezoelectric substrate 24. In FIG. 1 the SAW propagates in only one direction.
A reflector bank 30 (comprising a pattern of metal electrodes that are also referred to as elements or reflectors) some distance from the IDT 20 on one side (as illustrated in FIG. 1) or both sides of the IDT 20 generates reflected surface acoustic waves 32 back to the IDT 20 where they are converted to an RF echo signal 34 that is transmitted from the antenna 18 to the interrogator 10 for processing. The reflector bank 30 can launch a single frequency echo as used in a CDMA RFID (code division multiple access radio frequency identification) tag system or multiple frequencies such as used in an OFC system. Generally, the characteristics of the echo or reflected SAW are representative of physical parameters of the reflector array. For example, spacing of the reflectors of the reflector array, which affect the frequency and/or phase of the reflected SAW signal, are affected by a temperature of the substrate, which may in turn be affected by an ambient temperature of the region surrounding the SAW device.
FIG. 2 illustrates a prior art RF echo signal as generated by the SAW sensor system 8 of FIG. 1. Signal peaks represent the echo from each reflector bank, thus FIG. 2 depicts the echoes of a SAW sensor having three reflector banks.
In one embodiment of a SAW sensor, the substrate is bonded to a header material with bond wires connecting the SAW components to pins on the header. The SAW sensor further comprises a header lid for covering the SAW components.
FIG. 3 illustrates a prior art SAW sensor system comprising four SAW sensors 40-43 with RF interrogating signals generated by the interrogator 10, transmitted from a monopole antenna 45, and received by each of the SAW sensors 40-43. The user controller 11 communicates with the interrogator 10 over an Ethernet link 48.
FIG. 4 depicts the echoes from each sensor of the four-sensor system of FIG. 3, each echo or reflection labeled with the reference numeral of the sensor system that generated the echo.
In lieu of using a monopole or dipole antenna, such as the antennas 13 and 45 of FIGS. 1 and 3, respectively, a patch antenna can be used. The patch antenna and the SAW substrate may be mounted on the same supporting material with the patch antenna proximate the SAW substrate.